INT 21,37 - Get/Set Switch Character (Undocumented, DOS 2.0+)

	AH = 37h
	AL = 0	get switch character into DL;  some systems return "-"
	   = 1	set switch character to value in DL
	   = 2	read device prefix flag into DL;  returns DL = 0 indicating
		devices must be accessed using /DEV/device. A non-zero value
		indicates devices may be accessed without prefix
	   = 3	set device prefix flag, device names must begin with \DEV\.
	DL = new switch character (AL=1)
	   = 00  \DEV\ must preceed device names (AL=3)
	   = 01  \DEV\ is not neccesary in device names (AL=3)

	on return:
	AL = FF  illegal subfunction code specified
	DL = current switch character (AL=0)
	   = device availability (AL=2, always FF with DOS 4.x+)

	- subfunctions 0 and 1 were formerly available as a CONFIG.SYS
	  command in DOS versions before 3.x;	also supportedin the OS/2
	  compatibility box
	- subfunctions 2 and 3 were available in DOS 2.x only;	they have
	  no effect in DOS 4.x+
	- /DEV/ prefix is valid in DOS 2.x by default, setting the flag makes
	  it mandatory.  DOS internal commands like DIR, TYPE and DEL do not
	  recognize filenames the are identical to device names regardless of
	  the setting of the AVAILDEV flag.